A communication system, such as a multi-channel modulation system or an OFDM system, having a large envelope amplitude fluctuation has a large peak to average power ratio (PAPR). Hence, high efficiency is difficult to realize with most conventional power amplifiers. A Doherty type amplifier in which amplifiers operating at different bias points from one another are combined has been conventionally known as a method of solving this problem. Descriptions of the Doherty type amplifier may be found in the literature, for example, W. H. Doherty, “New high efficiency power amplifier for modulated waves”, Proceeding of the IRE, Vol. 24, No. 9, pp. 1163 to 1182, (1936).
A brief review of the Doherty type amplifier follows. Firstly, a Doherty type amplifier relating to a prior art will be described with reference to FIGS. 14 to 18.
FIG. 14 is a structural diagram of a Doherty type amplifier relating to a prior art.
FIG. 15 is a signal waveform diagram illustrating typical operation of the Doherty type amplifier shown in FIG. 14.
FIG. 16 is a graphical representation showing a relationship between output power range and power added efficiency (PAE) in simulation results for the Doherty type amplifier shown in FIG. 14.
FIG. 17 is a graphical representation showing a relationship between frequency range and power added efficiency (PAE) in the simulation results for the Doherty type amplifier.
FIG. 18 is a schematic diagram showing operating points of a carrier amplifier and a peak amplifier provided in the Doherty type amplifier.
A basic structure of the Doherty type amplifier is shown in FIG. 14. That is to say, a first amplifier (carrier amplifier), and a second amplifier (peak amplifier) are connected in parallel with each other. The first amplifier operates irrespective of the amplitude of an input signal to produce an output signal corresponding to the amplitude of the input signal. The second amplifier operates when the amplitude level of the input signal is equal to or larger than a certain threshold to produce an output signal corresponding to the level of the amplitude of the input signal above the threshold. When the amplitude of the input signal is small, the carrier amplifier operates in a linear region. The above-mentioned threshold is set near the amplitude level of the input signal corresponding to saturation power of the carrier amplifier in this operating region. Then, when the amplitude of the input signal is larger than the threshold, the output voltage of the carrier amplifier is clipped and thus the output waveform of the output voltage is distorted. However, the peak amplifier operates and supplies a current to the load so as to compensate for the clipped portion. As a result, distortion is reduced and at the same time, high power efficiency is maintained. FIG. 15 shows an example of amplitudes of signal waveforms in this case. As shown in FIG. 16, the Doherty type amplifier can realize high power efficiency for a wide output range.
Hereinafter, the principle of the operation of the Doherty type amplifier will be described in more detail. In general, when the peak amplifier and the carrier amplifier are connected in parallel with each other, a load voltage increases with a current supplied from the peak amplifier. Thus, when viewed from the carrier amplifier side, the load resistance appears to increase, leading to lower power efficiency. In the Doherty type amplifier, a ¼-wavelength transmission line, TL2 in FIG. 14, is connected between an output terminal of the carrier amplifier and an output terminal of the peak amplifier. A load R connected to an output side of the peak amplifier appears to the carrier amplifier to be Zo2/R (where Zo is the characteristic impedance of the transmission line) due to the ¼-wavelength transmission line. Although the load voltage increases with a current supplied from the peak amplifier, however, when viewed from the carrier amplifier side, the load resistance seemingly decreases. As a result, the efficiency of the carrier amplifier increases. Here, since the phase of a signal from the carrier amplifier is delayed by 90 degrees due to the ¼-wavelength transmission line on the output side, it is necessary to delay the phase of a signal through the peak amplifier by 90 degrees on the input side. A ¼-wavelength transmission line, TL1 in FIG. 14, serves this purpose.
In order that the circuit shown in FIG. 14 may operate as the Doherty type amplifier, a bias voltage or current is set in a carrier amplifier Amp1 so that the carrier amplifier Amp1 operates as a Class B amplifier, and a bias voltage or current is set in a peak amplifier Amp2 so that the peak amplifier Amp2 operates as a Class C amplifier. FIG. 18 shows operating voltage bias conditions of the carrier amplifier Amp1 and the peak amplifier Amp2. An offset bias voltage having the same magnitude as that of the input voltage amplitude right before the carrier amplifier Amp1 is saturated is applied to the peak amplifier Amp2.
The Doherty type amplifier according to the above-mentioned prior art realizes the power distribution of the input and the phase delay of 90 degrees, using a ¼-wavelength transmission line. Also, the Doherty type amplifier realizes the impedance conversion and 90-degree phase delay for the carrier amplifier output and power combination with the peak amplifier output, using another ¼-wavelength transmission line. In this prior art, however, the Doherty type amplifier can perform neither of the phase delay of 90 degrees nor the impedance conversion at a frequency greatly different from the central frequency of the amplifier. Hence, the Doherty type amplifier can show a high efficiency effect only for a narrow frequency band centered about the frequency corresponding to the ¼ wavelength. When the power-added efficiency (PAE) at 12 dB backoff is plotted in the form of a graph against frequency, an inverted-V curve as shown in FIG. 17 is obtained. Thus, the PAE becomes maximum at a certain frequency, and decreases as the magnitude of frequency difference from the certain frequency increases.
The present invention has been conceived in order to solve the above-mentioned problems. It is an object of the present invention to make the electric length of the output power combining circuit of a Doherty type power amplifier variable to realize a high efficiency for a multi-band or broad band.